Applicant claims priority under 35 U.S.C. xc2xa7119 of German Application No. 199 25 800.7 filed Jun. 3, 1999. Applicant also claims priority under 35 U.S.C. xc2xa7365 of PCT/DE00/01807 filed May 30, 2000. The international application under PCT article 21(2) was not published in English.
The invention relates to the field of electrical engineering/electronics. Objects that can be applied and are useful are components based on surface acoustic waves such as employed, for example in wide-band band-pass filters and in delay lines.
Converters for surface acoustic waves are known which are composed of groups of prongs mounted on a piezoelectric substrate. Such groups of prongs each are comprised of at least two prongs, and at least some of the groups of prongs are structured in such a manner that they are different from the other groups of prongs with respect to the wave amplitude they excite.
In a special embodiment (WO 97/10646), interdigital converters designed with a tapering structure are composed of groups of prongs each comprised of three prongs. Two of said prongs form a pair of prongs having no reflection, whereas the third prong of each group is a reflector prong. The spacing between the center lines of the reflector prong and the prong of the pair of prongs located adjacent to said reflector prong typically amounts to 3 pzg/8. In this connection, pzg represents the length of a group of prongs along a straight line extending parallel with the collector electrodes with a preset spacing from one of said collector electrodes. Each group of prongs consequently has a preferred direction with respect to the generated wave amplitude. A converter structure of this type is therefore referred to as single-phase unidirectional transducer, abbreviated as a SPUDT. If the width of the reflector prong amounts to pzg/4 or pzg/8, the groups of prongs are referred to as EWC- or DART-cells, respectively. As long as single-phase unidirectional transducers are not designed in the form of tapering structures, they are suited for filters with low insertion damping up to a band-width of about 1%. However, if single-phase unidirectional transducers are combined with the design principle of tapering structures, which is the case in reference [1] cited above, filters with low insertion damping can be realized even up to a bandwidth of up to at least 50%.
So as to be able to adjust the transmission behavior of the described filters in the desired manner, it is necessary to weigh the density of the wave amplitudes generated by the individual groups of prongs. The method of weighing the overlap, which is known from transversal filters, cannot be applied in the present case without serious drawbacks because the amplitudes have to be substantially continuous over the entire aperture of the participating transducers. In another special embodiment, which comes closest to the present invention, said problem has been solved by weighing the density of the gap (H. YATSUDA, IWEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, vol. 41, March 1997, pages 453-459 [2]).
Another method for weighing the density of the wave amplitudes generated by groups of prongs in such a manner that such amplitudes are substantially continuous over the entire aperture of the participating transducers, is described in DD 208 512 [3]. The converters employed in said reference are transducers which, together with a transducer with a weighed overlap, form transversal filters, and consequently enhance due to their weighed density the selection of the blockage, as compared to the use of uniform transducers. The groups of prongs of said transducers are arranged parallel with each other and consequently do not form any tapering structure. Each group of prongs is composed of two equally wide prongs. The substantially homogeneous but nonetheless weighed density of the amplitude across the entire aperture is assured in that some of the groups of prongs are subdivided in two subconverters having the same aperture, and in that these subconverters are electrically connected in series. The weighed density factor of one of the structured groups of prongs described in [3] amounts to xc2xd of the value in relation to an unmodified group of prongs. This is a consequence of the fact that two identical subconverters of a group of prongs are connected in series, which causes the voltage applied to a structured group of prongs to amount to xc2xd of the amount of the converter voltage.
It is common practice to determine by suitable methods with the help of the filter specification continuous density functions and to subsequently convert such functions into a gap density. The solution [2] has the drawback that the weighed density of the gap, which only permits weighed density factors of equal to 0 or xc2x11 in some cases, is too rough for permitting a continuous density function to be reproduced with adequate accuracy. The filter so implemented may consequently substantially deviate from the requirements of the filter specification especially in the blocking area even though the application of the continuous, weighed density function, which is of only theoretical significance in the present case, would have assured that the specified requirements are satisfied.
The invention is based on the problem of altering converters for surface acoustic waves of the known type in a manner such that also weighed density factors of other than 0 or xc2x11 can be realized in spite of the substantially homogeneous profile of the amplitude.
Viewed as converters of the known type are those converters that are composed of groups of prongs mounted on a piezoelectric substrate, where each group of prongs is comprised of at least two prongs and collector electrodes, whereby the arrangement of prongs, in its entirety, is forming a structure tapering in the direction of one of the collector electrodes.
The problem is solved with the converter for surface acoustic waves specified in claim 20. The dependent claims specify variations of the embodiment of the invention.
For solving the problem, some of the groups of prongs, which are designated as structured groups of prongs, are subdivided in the direction of the prongs in a number of subconverters and electrically connected in series, which causes such structured groups of prongs to be different from the remaining groups of prongs with respect to the wave amplitude they excite.
This solution permits combining the advantages offered by tapering converter structures with the application of discontinuous density weighing methods that permit finer, stepped density factors than the weighed density gap. If a structured group of prongs is subdivided into an xe2x80x9cNxe2x80x9d number of identical subconverters connected in series, the density factor of such a group of prongs is equal to xc2x11/N. In addition to the density factors 0 and xc2x11 characteristic of the gap density, the following density factors are consequently adjustable: for example xc2x11/2 and xc2x11/3.
The density weighing method described above, which is combined with the tapered alignment of the converter prongs, has little significance for converters without a tapering alignment of the prongs because the weighed overlap density is available in that case, which is normally applied as well. As opposed to the above, the application of the weighed overlap density is not useful with a tapering alignment of the prongs because it would destroy the advantages they offer. Therefore, applicable are only those density weighing methods that modify the electrical voltage on the prongs. The described density weighing method is capable of effecting such a modification. Even though it has little significance for converters without the tapering alignment of the prongs, the claimed combination of features offers in a surprising way an enhanced solution to the density weighing problem in connection with converters with a tapering alignment of the prongs.
The invention can be usefully realized as follows:
According to a first useful embodiment, all subconverters have the same aperture.
Furthermore, it is useful if all groups of prongs each contain two or three prongs.
It is especially useful if two prongs in each group form a pair of prongs, whereby the prongs of a pair of prongs are equally wide and are connected to different collector electrodes, and if such prongs are arranged in relation to one another in such a way that the pair of prongs is without reflection overall, and the third prong in each group is a reflector prong. Special embodiments of such an arrangement comprise groups of prongs that are designed as EWC- or DART-cells.
It is especially useful if two prongs in each structured group form a pair of prongs, whereby the prongs of a pair of prongs are equally wide and are connected to different collector electrodes, and if such prongs are arranged in relation to one another in such a way that the pair of prongs is without reflection overall, and the third prong in each group is a reflector prong. Special embodiments of such an arrangement comprise groups of prongs that are designed as EWC- or DART-cells.
Furthermore, if every group of prongs is comprised of a pair of prongs and a reflector prong, the reflector prong and one of the prongs of the pair of prongs of the structured groups of prongs may be connected in each subconverter in an electrically conductive manner so as to form a subgroup of prongs.
Moreover, it is useful if each subgroup of prongs is connected in each case in an electrically conductive manner to the prong of the following subconverter of the same group of prongs not belonging to a subgroup of prongs. However, it is possible also that at least one subgroup of prongs is connected in an electrically conductive manner to the subgroup of prongs of the following subconverter of the same group of prongs.
The approximation of the density to the continuous function for weighing the density of the amplitude or reflection factor can be supported if the width of the prongs belonging to a pair of prongs, or the width of the reflector prong in at least one group of prongs is different from the width in the remaining groups of prongs.
It is useful if structured groups of prongs are present with a varying number of subconverters each having the same aperture. On the other hand, it is possible also that all structured groups of prongs are subdivided in the same number of subconverters each having the same aperture.
Such a subconverter, however, may contain at least two neighboring groups of prongs as well, each having the same number of subconverters with the same aperture. It is useful in such a case if the subgroups of prongs of the neighboring groups of prongs are. connected to each other in an electrically conductive manner.
Finally, it is highly useful if a converter as defined by the invention is the input and/or output converter of a surface acoustic wave filter.
The invention is explained in greater detail in the following with the help of an exemplified embodiment and an associated drawing.